A typical “three-piece” railroad car truck comprises two parallel side frames connected by a bolster laterally spanning the distance between the side frames. Each end of the bolster includes at least one, although usually two, wedge-shaped pockets adapted to receive a spring-mounted friction wedge or friction casting.
The side frame to bolster connection design of three-piece trucks is generally characterized by a triangular friction wedge in contact with and contained by the bolster pocket on one side, a vertical surface of the side frame on another, and a spring on the third side. The connection is comprised of three load-bearing interfaces: a bottom surface, a front surface, and a back surface. The wedge surfaces are oriented in the shape of a right triangle with the bottom and front surfaces oriented at a right angle to each other, and the back surface oriented at an acute angle to the front surface. The wedge is oriented with the front surface vertical to allow sliding motion of the bolster relative to the side frame due to dynamic forces of the rail car body. The wedge back surface bears on a sloped face of the bolster pocket, which acts to direct the force of the spring from the bottom surface into the front surface of the wedge. As a result of the wedge configuration and orientation, a force balance is formed on the friction wedge, at the three interfaces, that is governed by the relative position and movement of the bolster to the side frame.
During use of the truck, most typically at high operating speeds, “hunting” is known to occur. The term “hunting” refers to the situation wherein one of the side frames gets ahead of the other side frame, which misalignment causes the bolster to rotate about a vertical axis from its ideal perpendicular orientation with respect to the side frames. This disorientation of the bolster leads to several problems. For one, the forces acting upon the bolster and side frame can cause relative lateral movement therebetween which, in turn, causes relative lateral movement between the friction wedge and the bolster pocket. Such movement can cause wear to the side walls of the pocket and/or the sides of the friction wedge, especially if the friction wedge is allowed to repeatedly, forcefully press or rub against the pocket.
Another problem caused by “hunting” is the tendency of the spring supporting the friction wedge to deflect from its ideal, vertical orientation. This deflection causes the friction wedge to rotate within the pocket, pressing an upper corner and the opposite lower corner of the wedge against opposite side walls of the pocket, creating a squeezing force that can wear the pocket and/or the wedge.
The ability of the truck to resist these unsquaring forces is referred to as its warp restraint or warp resistance. There are different types of friction wedges, each having different warp resistance characteristics. The different types of friction wedges can be generally categorized as either of unitary or combination construction and as either of a single-piece or split construction. A unitary friction wedge is cast as a single metal body, typically of iron or steel. On the other hand, in a combination friction wedge, a plate or insert is positioned between a support wedge body and the bolster pocket to provide the aforementioned back surface or otherwise modify the interaction between the support wedge body and the pocket. Use of a wear plate or insert is discussed in U.S. Pat. Nos. 3,559,589 to Williams; 4,426,934 Geyer; 4,974,521 to Eungard; 5,555,817 to Taillon, et al.; and 5,850,795 to Taillon, all of which are hereby incorporated herein by reference.
A friction wedge with a single-piece construction is a wedge configured to occupy the entirety of an associated bolster pocket. In contrast, when multiple wedges (typically two half-sized wedges that are usually supported by a single spring) are configured to be received in a single bolster pocket, it is often referred to as a split configuration. Both single-piece and split wedges may also be unitary or combination wedges, giving a wide variety of possible friction wedge configuration types. U.S. Pat. No. 6,895,866 to Forbes illustrates a number of different unitary/combination/single-piece/split friction wedges and is hereby incorporated herein by reference.
In general, known single-piece friction wedges will provide vertical damping and moderate squaring ability, but are slightly narrower than the associated pocket, allowing them to rotate in the bolster pocket. Consequentially, they do not provide maximum warp resistance. By comparison, split wedges provide vertical damping and a higher squaring ability by spreading away from each other in the bolster pocket to abut the side walls, thereby preventing rotation within the pocket. The split wedges are allowed to move up and down relative to each other to provide increased warp resistance. However, as described above, abutting the side walls of the bolster pocket can cause wear to the pocket and/or the friction wedge, so a friction wedge with a high squaring ability that also avoids contact with the side walls may be advantageous.